Unexpectedly, many meteorites, of every type, contain secondary minerals that must have formed through the initial mineral reacting with liquid water (‘aqueous alteration’). A recent dating study shows that the alteration took place very soon after the formation of chondrules and often during accretion of the incorporating bodies (Doyle et al. 2015). In some cases the minerals hydrated even before accretion. Water vapour suspended in space was wetting the grain surfaces and making them stickier, accelerating the process by which the grains coalesced. Another interesting point is that the amount of water tends to be in inverse proportion to the chondrule content [see part 3]. Some meteorites that lack chondrules consist entirely of aqueous minerals. Apparently, the source of the heat that prevented hydration was the chondrules themselves. If conditions were cool enough, hydration almost always occurred.
Hydrated minerals have also been detected remotely in comets and asteroids and directly recovered by NASA’s Deep Impact probe from the short-period comet Tempel 1. Comets, of course, contain copious amounts of water.
Interplanetary space seems to have been wet. Evidence for this doesn’t just come from asteroids and comets. All the terrestrial planets show signs of having once been drenched by water – even Mercury. One of the most astonishing findings of the Messenger mission to the planet was that in areas untouched by solar radiation (under the walls of high-latitude craters) water abounds. Deposits 50 m thick are thought to lie on the crater rims permanently in shadow. Ceres, the largest body in the asteroid belt, is estimated to be 50% ice and 50% rock by volume.
Perhaps the most surprising instance is Venus, the hottest planet in the Solar System. Although the planet today has a hot dry surface and is shrouded under clouds of carbon dioxide and sulphuric acid, the high ratio of deuterium to hydrogen in its atmosphere suggests that it once hosted a substantial ocean, subsequently evaporated (or blasted) away. Deuterium, an isotope of hydrogen, can combine with oxygen to produce a heavy form of water, and the inference is that ultraviolet radiation from the sun split the evaporated water into hydrogen, deuterium and oxygen. The lightest gas, hydrogen, escaped into space, as did most of the deuterium, but a proportion remained in the atmosphere. The heavier oxygen oxidised the crust.
Until recently, the Moon was believed to be devoid of water. Then in October 2010 it was announced that the LCROSS (Lunar Crater Observation and Sensing Satellite) mission had discovered larger quantities of water when it drove a spacecraft into a crater close to the permanently shadowed south pole. Five months after that, it was announced that millions of tons of ice were hidden deep within craters around the north pole. The polar ice had to be very ancient. Water was even discovered in volcanic melt inclusions – tiny pockets of magma trapped in the growing crystals while the magma was as yet unerupted – suggesting that the interior also contained appreciable amounts. Indeed, the proportions were similar to those within the Earth’s upper mantle. However, because of the difficulty of understanding where it might have come from, scientists continued to doubt that water was present on the surface. Hence it still made the news when in August 2018 a paper analysing data gathered by India’s Chandrayaan-1 probe in 2008-09 gave the first definitive proof of ice on the surface. The existence of water on the Moon can be doubted no longer.
Oceans of water cover most of the Earth’s surface, to an average depth of almost 4 km. According to the nebula hypothesis, Earth, like Venus, should not have had oceans to start with, since it lies within the ‘snow line’ within which the infant Sun’s heat – not to mention the postulated ocean of magma at its surface – would have prevented volatiles from condensing into liquid. Yet water has been abundant on or in the Earth from as far back as datable minerals can take us, in geological time as early as 4.4 billion years ago. At the beginning of the Archaean, around 3.9 billion years ago, the entire planet was under water, and it was to remain largely submerged for another 1300 million years (Flament et al 2008). Water has dominated the planet throughout its history.
Mars’s early history is no less puzzling. Its surface today is cold and dry, yet evidence of former water turns up wherever one looks. The ancient impact-gouged depression in its northern hemisphere once contained an ocean more than 400 metres deep, covering a third of its surface. Deltas and valley networks – conduits of water from the highlands – fringe the basin. Within the basin one can still make out the outlines of smaller craters whose walls were eroded by the ocean and whose floors received thick sheets of diluvial sediment. In other regions, the craters are surrounded by ejected material that formed splashes, showing that the ground possessed (or was shock-heated to) a mud-like consistency. The surface was wet when asteroids bombarded the planet. For many years, clouds continued to rain on the lowlands, repeating their cycles of evaporation, re-precipitation and runoff, until gradually the water seeped into the ground. Today the planet is colder and the water is locked up as subsurface ice.
Jupiter consists mostly of hydrogen and helium – the helium a product of nuclear fusion in the core when the speed of light was much faster than now. It is by far the largest planet in the solar system and was long thought to be dry. Recent data suggests that Jupiter has 2 to 9 times more oxygen than the Sun and hence abundant water. NASA’s Juno probe is currently attempting to verify this finding, with potential implications for water in the gaseous planets beyond: Saturn, Uranus and Neptune.
- its fragmentary nature, it is estimated to contain more than 100,000 objects over 50 km in size and, wildly contrary to computer models, quadrillions of objects 10–100 metres in size (Cooray 2006);
- the belt’s low overall density, this is not satisfactorily explained by the nebula hypothesis and is known as the ‘missing mass problem’, though the problem may be partly alleviated by the quantity of the 10-100 metre-size objects;
- the ‘surprisingly high level of dynamical excitation’ of the objects – they have highly elliptical orbits at various angles to the ecliptic plane, not, as expected, circular orbits all close to the plane;
- the existence of more such bodies, known as the “scattered disc”, that extend in similarly erratic orbits beyond the Kuiper Belt and are essentially a continuation of it.
The largest Kuiper Belt Objects (KBOs) are Pluto, Makemake and Eris, all classified as dwarf planets. Several others are suspected to be of similar size. The icy moons of Neptune and Uranus may also have been former members of the Kuiper Belt, as may some of the Centaurs. As with the asteroid belt, the vast number of these objects is thought to reflect the outcome of collisions between larger bodies. Thus the present state of the Kuiper Belt does not reflect its primeval state, and its more recent history may be one of disaggregation rather than aggregation.
The composition of the KBOs has to be inferred from their surface composition. This is not straightforward, since a variety of events and influences, such as interaction with the interstellar medium and polymer-producing cosmic rays, has complicated the chemistry. Pluto, about one third the size of Earth’s moon by volume, is one third water and two-thirds rock. The thousands of impact craters dotting its surface bear graphic testimony to the ‘dynamical excitation’ of the objects formerly in its vicinity.
In simple terms, the surfaces of the largest bodies are mainly nitrogen and methane (CH4) whereas the surfaces of the small to medium-sized objects are mainly water-ice. Most of the smaller bodies are fragments of larger ones and therefore younger. Some of the water ice is crystalline and must have formed in temperatures well above those now prevailing. This may not have been long ago, since cosmic rays will reduce crystalline ice to an amorphous state within 0.1–1.0 million years.
The atmospheres of Jupiter, Saturn, Uranus and Neptune also contain substantial amount of ammonia (NH3), methane and water (as do all moons large and cold enough to retain such volatiles).
In view of the problems associated with the nebula hypothesis, one should at least consider whether a creation-based approach might not offer a better interpretation. Many pre-scientific peoples had a tradition that a celestial ocean existed above the terrestrial one. The Egyptians, for example, visualised the sun as travelling through the sky in a boat. The creation myth of Babylon, Enuma Elish, related that the goddess of the deep split in two to form an upper ocean and a lower one. According to the Hebrews, the space encompassing the solar system – termed the firmament – was created by separating the primordial deep into two bodies of water, one above the firmament and another under it. The latter was a subterranean body that watered the Earth’s surface from below.
Combining ancient tradition and modern astronomical knowledge, we could surmise that these celestial waters initially existed in the gas phase, forming a protective, nebulous, slowly rotating circumambient shell not unlike the spherical shape postulated for the ‘Oort cloud’. Because of the heat emanating from the Milky Way’s once luminous nucleus, interstellar space was originally much warmer than today. Over time, much of this water diffused inwards under the influence of the Sun’s gravity, the shell contracting into an annular disc. By 4.567 Ga ago in geological time interplanetary space may have hosted a substantial volume of water. In the course of diffusion, some of this water showered onto the planets – hence the large volume of water attracted, for example, by Mars in its Noachian period. By then, water seems to have been ubiquitous through the solar system. Further out, cooling and electrostatic sticking caused the droplets to consolidate into small bodies of ice. The Kuiper Belt can be understood as a mixture of frozen water deriving from the circumambient shell and the exploded remains of a rocky planet.
Consistent with the tradition that interplanetary space was created by dividing a once single body of water, isotopic ratios support the idea that the Earth’s ocean and the water locked up in comets, including Oort Cloud comets, had a common origin (Lis et al. 2019).
According to Genesis, the original Earth was destroyed in a terrible cataclysm, popularly known as ‘Noah’s Flood’. There were two agents of destruction. One was the springs of the great subterranean deep, which suddenly exploded and flooded the whole planet; even mountains were submerged. The other was the rain that for 40 days fell through the ‘windows of the heavens’. Apparently as a consequence of the rain, all life was blotted out from the face of the ground (Gen 7:4).
The rain is problematic, for it is impossible to model an atmosphere that might have held that amount of water, and it is difficult to see how 40 days of rain, however torrential, could have drowned all mountains and resulted in life’s total obliteration. Genesis, moreover, says that up to that point rain was unknown: the Earth’s surface was watered from below, not above. While some water must have evaporated into the atmosphere from lakes and inland seas, as well as from the ground itself, this does not seem to have been a major component of the hydrological cycle.
On the other hand, it might not have been water that rained on the planet. In the story about Sodom and Gomorrah the text says that God rained sulphur and fire on the cities. In Psalm 11 he is pictured as raining down coals. In the book of Joshua he brings down a ‘hail’ of ‘large stones’, i.e. meteorites, on Israel’s enemies. It therefore seems better to understand the rain in the time of Noah as an onslaught of asteroids. Indeed, just such a bombardment is known to have pummeled the Earth early in its history. Whether this occurred immediately before the beginning of the Archaean or over a longer preceding period is debated, but we know that Earth was bombarded by asteroids from dating the largest impact craters on the Moon’s surface, in astronomical terms just a stone’s throw away. Asteroids certainly would have obliterated life on Earth.